msm_fb: display: Do clock control within mdp4_hw_init

Since mdp4_hw_init() access mdp registers, mdp clock control
within mdp4_hw_init() is necessary.

CRs-fixed: 455506
Signed-off-by: Kuogee Hsieh <khsieh@codeaurora.org>
Change-Id: I768f468314fc920029b4c88e49ef6a74f6b74f58